Asset: 'test_export_assets' flakes

[tseaver]

/cc @gaogaogiraffe (test added in PR googleapis/google-cloud-python#8613, configuration updated in googleapis/google-cloud-python#8627)

From this Kokoro failure today:

___________________ TestVPCServiceControl.test_export_assets ___________________
self = <test_vpcsc.TestVPCServiceControl object at 0x7fe8f53fa9d0>
    @pytest.mark.skipif(
        PROJECT_INSIDE is None, reason="Missing environment variable: PROJECT_ID"
    )
    @pytest.mark.skipif(
        PROJECT_OUTSIDE is None,
        reason="Missing environment variable: GOOGLE_CLOUD_TESTS_VPCSC_OUTSIDE_PERIMETER_PROJECT",
    )
    def test_export_assets(self):
        client = asset_v1.AssetServiceClient()
        output_config = {}
        parent_inside = "projects/" + PROJECT_INSIDE
        delayed_inside = lambda: client.export_assets(parent_inside, output_config)
        parent_outside = "projects/" + PROJECT_OUTSIDE
        delayed_outside = lambda: client.export_assets(parent_outside, output_config)
>       TestVPCServiceControl._do_test(delayed_inside, delayed_outside)
tests/system/test_vpcsc.py:67:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
delayed_inside = <function <lambda> at 0x7fe8f5348b90>
delayed_outside = <function <lambda> at 0x7fe8f5348c08>
    @staticmethod
    def _do_test(delayed_inside, delayed_outside):
        if IS_INSIDE_VPCSC.lower() == "true":
>           assert TestVPCServiceControl._is_rejected(delayed_outside)
E           assert False
E            +  where False = <function _is_rejected at 0x7fe8f3031de8>(<function <lambda> at 0x7fe8f5348c08>)
E            +    where <function _is_rejected at 0x7fe8f3031de8> = TestVPCServiceControl._is_rejected
tests/system/test_vpcsc.py:47: AssertionError

[busunkim96]

Closing since this is stale.